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Skeletal RadiologyJournal of the International SkeletalSociety A Journal of Radiology,Pathology and Orthopedics ISSN 0364-2348 Skeletal RadiolDOI 10.1007/s00256-017-2831-2

Gemelli-obturator complex in the deepgluteal space: an anatomic and dynamicstudy

Ramon Balius, Antonio Susín, CarlesMorros, Montse Pujol, Dolores Pérez-Cuenca & Xavier Sala-Blanch

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SCIENTIFIC ARTICLE

Gemelli-obturator complex in the deep gluteal space: an anatomicand dynamic study

Ramon Balius1,2 & Antonio Susín3& Carles Morros4 & Montse Pujol1,5 & Dolores Pérez-Cuenca4 & Xavier Sala-Blanch6,7

Received: 7 July 2017 /Revised: 5 October 2017 /Accepted: 16 November 2017# ISS 2017

AbstractObjective To investigate the behavior of the sciatic nerve during hip rotation at subgluteal space.Materials and methods Sonographic examination (high-resolution ultrasound machine at 5.0–14MHZ) of the gemelli-obturatorinternus complex following two approaches: (1) a study on cadavers and (2) a study on healthy volunteers. The cadavers wereexamined in pronation, pelvis-fixed position by forcing internal and external rotations of the hip with the knee in 90° flexion.Healthy volunteers were examined during passive internal and external hip rotation (prone position; lumbar and pelvic regionsfixed). Subjects with a history of major trauma, surgery or pathologies affecting the examined regions were excluded.Results The analysis included eight hemipelvis from six fresh cadavers and 31 healthy volunteers. The anatomical study revealedthe presence of connective tissue attaching the sciatic nerve to the structures of the gemellus-obturator system at deep subglutealspace. The amplitude of the nerve curvature during rotating position was significantly greater than during resting position. Duringpassive internal rotation, the sciatic nerve of both cadavers and healthy volunteers transformed from a straight structure to acurved structure tethered at two points as the tendon of the obturator internus contracted downwards. Conversely, external hiprotation caused the nerve to relax.Conclusion Anatomically, the sciatic nerve is closely related to the gemelli-obturator internus complex. This relationship resultsin a reproducible dynamic behavior of the sciatic nerve during passive hip rotation, which may contribute to explain thepathological mechanisms of the obturator internal gemellus syndrome.

Keywords Deep gluteal syndrome . Gemelli-obturator internus complex . Sciatic nerve . Obturator internal gemellus syndrome

Introduction

The sciatic nerve arises from the lumbosacral plexus, fromroots L4– S2. This nerve typically runs from its origin, at thegreater sciatic foramen, to the popliteal fossa, where it dividesinto two branches. Along this route, two areas with clearlydefined pathways can be distinguished: one in the deep glutealspace, and the other at the level of the thigh. In the glutealspace, the sciatic nerve is protected from the sacrum by thepyramidal muscle, and later from the ischium by the gemellusmuscles (superior and inferior), the obturator internus, and,finally, the quadratus femoris muscle. This deep pathway tothe gluteus maximus is mainly characterized by the nerve’scurved layout until reaching the second part of the route, at thelevel of the thigh [1, 2]. Upon reaching the thigh, the sciaticnerve takes on a rectilinear route, deep into the long section ofthe biceps femoris.

Due to anatomical variations, repeated microtraumas, orjust the passing of years (aging), characteristic clinical cases

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s00256-017-2831-2) contains supplementarymaterial, which is available to authorized users.

* Ramon Baliusramonbaliusmatas@gmail.com

1 Consell Català de l’Esport, Generalitat de Catalunya,Barcelona, Spain

2 Sports Medicine Department, Clínica Diagonal, Barcelona, Spain3 Math Department, UPC-BarcelonaTech, Barcelona, Spain4 Anesthesiology, Reanimation and Pain Therapy, Clínica Diagonal,

Barcelona, Spain5 Facultad de Fisioteràpia, Universitat Internacional de Catalunya, Sant

Cugat del Vallés, Spain6 Anatomy and Embryology Unit, Faculty of Medicine, Universitat of

Barcelona, Barcelona, Spain7 Department of Anesthesiology, Hospital Clínic, Barcelona, Spain

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occur due to nervous dysfunction, like, for example, second-ary pseudo-sciatica, or dysfunction caused by entrapment ofthe sciatic nerve at the level of the piriformis muscle, knownas pyramidal syndrome [3–5]. Recently, it has been observedthat the sciatic dysfunction in the pelvis was not only causedby different types of conflict with the pyramidal muscle, butthere were also conflicts at other levels, giving rise to theconcept of deep gluteal syndrome [1, 2, 6]. One of thesecauses is sometimes attributed to the gemelli-obturatorinternus complex [7–9].

On the other hand, there is an increasing interest inneurodynamics as a therapeutic instrument and to preservehealth. Currently, there are neurodynamic assessments guid-ing rehabilitators to perform preventive treatments focused onthe major nerves [10]. Thus, different studies have assessedhow articular movements affect the normal biomechanics ofthe sciatic nerve in the thigh [11–17] or the pelvis [18].However, these studies were exclusively focused on flexion-extension movements of the coccyx-femoral joint.

Given the anatomical layout of the sciatic nerve in thegemelli-obturator internus complex, and based onneurodynamics previous studies showing Bgliding^ of thisnerve during the flexion-extension of the hip, we attemptedto assess the sciatic nerve’s behavior at this level with hiprotation maneuvers. The hypothesis was that the gemelli-obturator internus complex is closely related to the sciaticnerve, thus providing a specific, reproducible and constantbehavior for these neuromuscular structures during internaland external rotations of the hip.

Materials and methods

Anatomical and US studies in cadavers

The study included fresh cadavers of both adult men andwomen, with no surgical history of lumbar spine, pelvis orhip, and without history of inflammatory medical pathologies(osteoarthritis). The study on cadavers was approved by theEthics and Research Committee of the School of Medicine atthe University of Barcelona, and carried out in the HumanAnatomy lab of the School of Medicine at the University ofBarcelona. The cadavers were kept at a temperature of 4 °Cfor 36 h, until reaching the usual screening results for thesafety of the cadaver handlers. The cadavers were placed inprone position, with a 10-cm cushion located in the pelvicarea, causing a 10–15° flexion of the coccyx-femoral joint,with the pelvis fixed in a support. The cadavers were kept inthe dissection lab, at a room temperature of 22–23 °C, for aminimum period of 4 h before the beginning of the study.

An ultrasound of the sciatic nerve’s behavior was per-formed during passive mobilization maneuvers of the limb.

Ultrasound scanning was performed with an Aplio 500(TUS-500 5.0 Platinum Series, manufactured by ToshibaMedical Systems Corporation in Nasu, Japan) using a high-frequency linear array probe (PLT 1005BT), 5.0–14MHz fre-quency range. Most commonly used 2D frequency was dif-ferential harmonic of 14 MHz (diffTHI 14 MHz). The depthused was 5 cm, with a single focus at 1.8 cm and a dynamicrange of 65 dB. A sonographic examination of the long-axissciatic nerve was performed. Internal and external rotations ofthe hip were performed with the knee at 90° flexion, while thesciatic nerve was observed by ultrasound in order to evaluateits behavior. The scans were performed by one of the authors(R.B.), with over 23 years of experience in musculo-skeletalsonography. Ultrasound images obtained were saved in imageand video formats.

After obtaining the images, the corresponding anatomicaldissection was performed (Fig. 1). A dissection in planes ofthe specimens was performed by two experienced anatomists(CM and XSB). The skin and superficial fascia of the glutealarea were removed first, followed by sectioning of the gluteusmaximus lateral third, about 3–4 cm of its femoral insertion.By removing the superficial planes, the sciatic nerve was freedfrom its posterior and internal relations and attachments basi-cally through the inferior gluteal nerve and vessels, and fromthe posterior skin nerve of the thigh. Superior relationships(with the superior gluteal nerve and vessels) as well asmean-caudal relationships (with the nerve and pudendal ves-sels) were kept intact. Internal and external rotation maneu-vers of the hip were subsequently performed, similarly tothose carried out with ultrasound. Nerve movements wereassessed in relation to its muscle/tendon support in thegemelli-obturator internus complex. Anatomical images

Fig. 1 Anatomical dissection of the gluteal area. a Superficial glutealarea. b Deep gluteal space. GM gluteus maximus muscle, IT ischialtuberosity, lhBF long head of biceps femoris. P piriformis muscle, SGsuperior gemellus muscle, OI obturator internus muscle, IG inferiorgemellus muscle, QF quadratus femoris muscle. Arrow pathway of thesciatic nerve

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obtained were photographed, and the sciatic nerve movementwas filmed in direct view.

Healthy volunteer study

Participants

The study was approved by the Clinical Research EthicsCommittee of the Catalan Sports administration, and informedconsent was obtained from all participants. A high-resolutionultrasound was used to assess sciatic behavior in thesubgluteal space of a group of young athletes. To be eligible,participants must be healthy, over 18 years old, and must nothave pelvic and/or hip pathologies or symptoms indicative ofsciatic nerve dysfunction. Participants were excluded if theyhad a history of major trauma or surgery to the lumbar, hip,gluteal or hamstring origin regions; a positive straight leg raisetest; or sciatic nerve impairment. Participants were also ex-cluded if they had any pathology that might alter the functionof the nervous system.

Imaging

There is absolute consensus about the use of US for the dy-namic assessment of nerves [10, 14]; moreover, in order tostudy healthy population, the use of a nonionizing radiationsystem is an ethical advantage.

Movement of the sciatic nerve was assessed in deep glutealarea, distal to inferior edge of piriformis muscle and proximalto superior edge of quadratus femoris muscle. The changes inthe shape of the sciatic nerve at the beginning and end ofdifferent neural mobilization exercises have been quantifiedin terms of the curvature of the nerve in longitudinal captures.Initial short-axis imaging of the posterior buttock allowed lo-calization of the sciatic nerve. Once identified, the probe wasrotated approximately 90° in order to find the sciatic nerve inlong axis. A sonographer with 23 years of experience (RB)performed all ultrasound scans. A high-resolution ultrasoundmachine was used (TUS-A500 BAplio 500^ 5.0 Platinum,manufactured by Toshiba Medical Systems Corporation inNasu, Japan), with a high-frequency linear array probe (PLT-1005BT linear array, range of frequency 5.0–14 MHz). Presetcharacteristics were a depth of 5 cm, with single focus at1.8 cm and dynamic range of 69%. A video of the nervemovement was recorded for each exercise trial. The videosequence had a capture rate of 30 frames per second.

Hip mobilization exercises and its evaluation

The volunteer was placed on a stretcher in prone position andthe pelvis strapped to the bed, leaving the buttocks free inorder to fix the lumbar and pelvis regions during mobilizationof the hips.

Each participant performed two different motions, namely,a passive external hip rotation and a passive internal hip rota-tion.While these actions were carried out, sciatic behavior andrange of motion (ROM) were evaluated by long-axis ultra-sound. Each maneuver was recorded three times, choosingthe one best visualized by ultrasound.

Ultrasound image analysis

In each recorded video, two representative frames (beginningand end of the exercise) where manually selected, and the sciaticnerve was manually segmented on the images by a senior ana-lyst (Fig. 2a, Fig. 2b). This way, for the longitudinal captures, thenerve appears as a band in the ultrasound images (Fig. 2c, Fig.2d). We used MATLAB (The MathWorks Inc., Natick, MA,USA) software to first compute the medial axis of this band asthe representative curve of the nerve, and then we computed itscurvature. Curvature κ of a point P = [x(t), y(t)] belonging to acurve is mathematically defined as the inverse of the radius ofthe tangent circumference to the curve in this point [19]. It canbe computed from the coordinate derivatives using the formula:

κ ¼ x0y

0 0−x0 0y0

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

x02 þ y02� �3

r

The curvature of a curve is a good descriptor of the localshape of a curve, and it allows us to compare the shape of thesciatic nerve at the beginning and at the end of the motion. Acurvature value κ = 0 is associated to a flat curve (a straightline) and non-zero values correspond to proper curves.Moreover, positive and negative curvature values correspondto convex or concave parts of the curve, respectively.

Due to possible misalignments duringmotion recording, ananchor point, corresponding to a selected point of the posteriorpart of the acetabulum, has been used in order to align thesciatic nerve in the two images.

Results

Anatomical and US studies in cadavers

A dissection of eight hemipelvis from six fresh cadavers (fourmen and two women, aged 64–82) was performed. Thesonologist noted that, in all cadaveric specimens, visualizationof the sciatic nerve was enough in order to control its behaviorduring internal and external rotations. In all cases, there wasevidence of muscle tension during internal rotation. The ten-don of the obturator internus muscle showed a most striking,constant depression, which was identified on all 8 samplesassessed. This fact caused the sciatic nerve to curve, especially

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when intersecting the tendon of the obturator internus—Fig. 3a and Video 1 of the electronic supplementarymaterial (ESM). Furthermore, it was also observed that thesciatic nerve suffered another (less significant) angulation atthe distal border of the inferior gemellus (between this and the

quadratus femoris muscle). On the other hand, the nervecorrected itself through external rotation movements, with ob-jectification of its loss of tension (Fig. 3b and Video 1 of theESM).

The elevation of the nerve indicated the presence of con-nective tissue joining the nerve with deep tissues. In particular,the constant presence of a connective anchorage at the level ofthe sciatic nerve with the tendon of the obturator internus wasobjectified. This anchorage, although constant, varied as anentity, having a very close relationship in some samples, butnot so much in others (Fig. 4 and Video 2 of the ESM). Noother remarkable connective support was observed at the levelof the two gemellus muscles.

Healthy volunteer study

A total of 39 volunteers were evaluated. Of these, 8 individ-uals were excluded due to difficulty to correctly visualize thedeep gluteal space with an ultrasound, so the sample wasreduced to 31 cases (14 men and 17 women) with a meanage of 21.1 ± 2.7 years, a height of 173.5 ± 3.8 cm, a weightof 65.7 ± 6.6 kg and a BMI of 20.9 ± 3.5.

Instead of studying, for instance, the elongation of the sci-atic nerve, we focused on the description of the shape of thenerve with rotation of the hip. The longitudinal motion of the

Fig. 2 US study of the sciatic nerve’s behavior during internal rotation (aand c) and external rotation (b and d). Each image includes the initialframe (left) and last frame (right). The sciatic nerve is shown in yellowand the anchor point in red (c and d). Also the sciatic nerve is shown as anechoic and fibrillar tubular structure (a and b). During internal rotation (aand c), the sciatic nerve is tractioned at the level of the obturator internus

(left dotted arrow) and of the inferior gemellus with the quadratus femorismuscle (right dotted arrow). SG superior gemellus muscle, (*) obturatorinternus muscle, IG inferior gemellus muscle, QF quadratus femorismuscle. Notice visualization of the connective anchorage of the tendonof the obturator internus with the sciatic nerve, especially during internalrotation of the hip (continuous arrow)

Fig. 3 Dynamic study of the sciatic nerve with external rotation of the hip(a) and internal rotation of the hip (b). Notice how the sciatic nerve (*)corrects itself during external rotation, and curves at two levels duringinternal rotation (arrows). P piriformis muscle, SG superior gemellusmuscle, OI obturator internus muscle, IG inferior gemellus muscle, QFquadratus femoris muscle

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sciatic nerve was recorded in order to understand how theglobal shape of the nerve is conditioned by the anatomicalstructures involved in the motion.

It was observed how, during the passive movement of in-ternal rotation, the sciatic nerve assumed a typical curvature,caused by angulation of the nerve when intersecting the ten-don of the obturator internus. Occasionally, a small and lesssteep angulation was also observed at the distal border of theinferior gemellus (Fig. 2a and Video 3 of the ESM. Duringpassive maneuvers of external rotation, the sciatic nerve wasobserved to correct its layout above the gemelli-obturatorcomplex (Fig. 2b and Video 3 of the ESM).

It was decided to quantify the deformation of the curvefrom the initial rest shape to the final rotated shape of thenerve. When comparing the initial and final curvature valuesfor these two curves, significant differences are observed(Fig. 5) in terms of amplitude and mean values for each curve.In Fig. 5, all curvature values were plotted for both curves andall participants. Red points are associated to initial shapes andblue points to final shapes. On the right column, red points canbe seen concentrated near zero values (quite flat), while bluepoints reach higher values, meaning that final curves are dif-ferent and less flat than at the beginning.

Statistical boxplot of the results are presented in Fig. 6,where the curvature for the initial shape for all 31 participantsare compared to the final shape obtained after each motion.The units for the obtained numerical values are expressed interms of image coordinates (not in metric system) because thestudy was mainly interested in pointing out the differences

between the shapes of two curves (initial and final). Fromthe computed Wilcoxon p-value a significance difference(p < 0.05) was obtained in all the motions except for the iso-metric internal rotation motion.

Discussion

Our anatomical article shows the presence of connective tissueattaching the sciatic nerve to muscle/tendon structures of thegemellus-obturator system at a deep gluteal level. The sciaticnerve has attachment systems that keep anatomical relation-ships stable in the buttocks [20]. These support mechanismsare formed by collateral branches of the sciatic nerve (themostwell-known are superior gluteal nerve, inferior gluteal nerve,posterior skin nerve of the thigh) [1]; but also by the nervesand vessels of the pyramidal, gemellus, obturator internus andquadratus femoris muscles (superior and inferior gluteal arter-ies with terminal ischial branch). These attachment systemsare associated with the paraneural connective tissue, whichacts as a Bgliding system^ with adjacent tissues or muscles(with its corresponding perimysium). Thus, the skeletal-muscular system acts as a neural container, and forms a me-chanical interface for the nervous system [10]. The connectivejoint between the tendon of the obturator internus and thesciatic nerve described in this article probably has a funda-mental role in maintaining the pelvic curved pathway of thenerve and is the last significant anchorage from which the

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Fig. 5 Curvature values for the sciatic nerve: red dots are associated to thestarting position of the nerve, and blue dots to the final position after themotion. In each column, R (right) and L (left) leg rotation motions areshown. More amplitude in the curvature values is found on the rightcolumn corresponding to dynamic internal rotation

Fig. 4 Anatomical study of the relationship between the sciatic nerve andthe obturator internus. The sciatic nerve (*) is separated by two threadsthat traction it, enabling us to see the connective anchorage of this nervewith the tendon of the obturator internus (arrows). SG superior gemellusmuscle, OI obturator internus muscle, IG inferior gemellus muscle, QFquadratus femoris muscle

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nerve assumes a straighter route. Thus, this connective an-chorage acts as a real stabilizer of the neural pathway,preventing any lateral movements. In the cadavers study, theconnective anchorage was constant but with varied signifi-cance. Sometimes, the nerve was observed to be stronglyjoined to the tendon and, other times, this joint, although stillpresent, was less important. This fact could explain why Fillerand Gilmer-Hill [21] observed that sometimes the sciaticnerve did not touch the tendon of the obturator internus, butother times it was entrapped by it.

To assess the subgluteal space with precision, a high-endultrasound machine was used that is able to easily visualizedeep anatomical areas in a group of athletic, young and thinindividuals (BMI of 21.4 ± 2.4 kg/m2). This way, the sciaticnerve’s behavior during passive mobilization of internal andexternal rotations of the hip could be easily objectified.Several times, it was even possible to objectify the connectivejoint between the tendon of the obturator internus and thesciatic nerve (Fig. 4 and Video 3 of the ESM).

Neurodynamic characteristics of the sciatic nerve havebeen studied by several authors, virtually always at the levelof the thigh—Ellis et al. [13] assessed the extension of theknee combined with cervical flexion-extension andCoppieters et al. [15] assessed mixed movements of flexion-extension of the hip and/or knee. Sciatic behavior has also

been studied during the Straight Leg Raise Test [16, 17, 22].As already mentioned, all studies have focused on differentlevels of articular flexion-extension movements, but the sciat-ic nerve’s behavior during rotation movements of the hip hasnever been assessed. Hall et al. [23] observed that cervicalflexion associated with raising a straight leg did not showany significant movement of the sciatic nerve with hip flexion.These authors have concluded that cervical flexion did nothave a significant effect on neural tissue’s compliance duringincrease of straight leg. This fact was later confirmed by Elliset al. [13] when failing to observe any differences in the sciaticnerve excursion during mobilization of the knee alone (2.6 ±1.4 mm) or mobilization of the knee with cervical flexion (2.6± 1.5 mm); furthermore, isolated tension applied at cervicallevel caused minimal excursion of the sciatic nerve (−0.1 ±0.1 mm). The anchorage observed in the obturator internuscan contribute to override the proximal transmission of neuraltension from the cervical rachis to the thigh.

During passive maneuvers of internal rotation of the hip,the tendon of the obturator internus is contracted downwards(anterior position). The sciatic nerve anchored to this structurefollows it, tracing a characteristic curvature that can be ob-served both in healthy volunteers and in the cadavers study(Fig. 7a). During passive maneuvers of external rotation, thetendon of the obturator internus loses tension, allowing the

Fig. 6 Statistical boxplot representation of the curvature values for thesciatic nerve for all the participants. For each type of motion, curvature ofthe starting (left boxplot) and final shape of the nerve (rigth boxplot) isshown. Each boxplot represent the distribution of the data in quartiles and

shows the median value (red line). The p-values of the Wilcoxon pairedrank test for equality of medians between starting and final shape arereported

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sciatic nerve to relax and assume a straighter appearance (Fig.7b).

This characteristic and reproducible dynamic behavior ofthe sciatic nerve could be involved in pathological processes;thus, it could be related with the obturator internus/gemellussyndrome [1, 2, 7, 24]. Clinical doctors must include thissyndrome in the differential diagnosis of non-discogenic sci-atica. As in the case of the pyramidal syndrome, the diagnosisof the obturator internus/gemellus syndrome is usually per-formed by ruling out other possible causes of sciatic pain [2,24]. Murata [7] recommends to orient this diagnosis using thePACE or Freiberg tests at a fast pace. Repeated and fast ante-rior traction and curvature of the sciatic nerve, caused by thetendon of the obturator internus, justifies a continuous shear-ing of the sciatic nerve produced by these tests.

Due to the proximity and similarity between the structureand function of the pyramidal muscle and the obturatorinternus, many diagnoses and treatments for the pyramidalsyndrome also affect the obturator internus/gemellus syn-drome, with which it is confused [21]. In one anatomicalstudy, the tendon of the piriformis muscle was found to blendwith the tendon of the obturator internus in 48 of 112 cases [9].This indicates a strong interaction between the piriformis mus-cle, the obturator internus and the sciatic nerve, and a possibleentrapment point. On the other hand, the sciatic nerve runsunder the belly of the piriformis muscle and over the gemellisuperior-obturator internus complex, causing a scissor effectbetween the two muscles that can be a source of entrapment[1, 2, 24]. This scissor effect can increase during internal ro-tation of the hip, if it is considered that the sciatic nerve’sconnective anchorage to the obturator internus tractions it fur-ther downward (anterior position). Sometimes, this traction ispart of a few neurological tests for the diagnosis of deep glu-teal syndrome, involving passive internal rotation, like theFreiberg and FAIR tests [2].

On the other hand, obturator internus muscle spasms causesciatica, often due to irritation or entrapment of the nerve ofthe obturator internus [9, 25]. This nerve exits through thegreater sciatic hole, between the sciatic and the pudendalnerves, branches out into retrosciatic space, and reaches thelesser sciatic opening, innervating the obturator internus mus-cle [21]. If the connective anchorage of the sciatic nerve to thetendon of the obturator internus is considered, this muscle’sconstant contraction could prevent the sciatic nerve frommov-ing naturally and cause pain at this level.

This study shows several limitations. The sciatic nerve wasfreed from its posterior anatomical attachments (gluteusmaximus, as well as vessels and inferior gluteal nerve, andposterior skin nerve of the thigh), which might modify theshape assumed by the nerve during rotations of the hip.Lack of muscle tone in the anatomical sample would alsoaffect this deformity and, finally, rotations of the hip wererestricted by the advanced age of the cadavers. However, theselimitations probably tend to underestimate, instead of overes-timate, the movement. Regarding the sample of healthy vol-unteers, they were all young muscular individuals, so theshape assumed by the nerve could overestimate the effect withrespect to that observed in general population. In this sense,new studies focused on older populations with different phys-ical conditions are necessary to confirm the neurodynamicbehavior described in this study and estimate the significanceof the effect.

In conclusion, our study provides evidence about the ana-tomical relationship between the sciatic nerve and the gemelli-obturator internus complex, as well as about the dynamic be-havior of the characteristic and reproducible sciatic nerve dur-ing passive rotation maneuvers of the hip. In the future, thesefindings might have an interesting role in the differential di-agnosis of deep gluteal syndrome. However, it is necessary toperform broad population studies to prove the presence of this

Fig. 7 Illustrative diagram of the sciatic nerve’s behavior during hiprotations. During external rotation, the tendon of the obturator internuslacks tension, and the sciatic nerve relaxes, assuming a straighterappearance. During internal rotation, the tendon of the obturator

internus and gemelli tightens, followed by the sciatic nerve, whichsuffers a curvature. P Piriformis muscle, SG superior gemellus muscle,OI obturator internus muscle, IG inferior gemellus muscle, QF quadratusfemoris muscle, (*) Sciatic nerve

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dynamic behavior in other population groups, both healthyand with pathologies.

Acknowledgements To Iñigo Iriarte (ARS Medica, Bilbao, Spain) forinterpreting and doing the drawings and for Luis Vidal PhD for the man-agement of the article.

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict ofinterest.

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